Re: [v2 1/1] i2c: dev: prevent ZERO_SIZE_PTR deref in i2cdev_ioctl_rdwr()

[Alexander Popov <alex.popov@linux.com>]

On 19.04.2018 16:49, Uwe Kleine-König wrote:
> On Thu, Apr 19, 2018 at 03:29:22PM +0300, Alexander Popov wrote:
>> i2cdev_ioctl_rdwr() allocates i2c_msg.buf using memdup_user(), which
>> returns ZERO_SIZE_PTR if i2c_msg.len is zero.
>>
>> Currently i2cdev_ioctl_rdwr() always dereferences the buf pointer in case
>> of I2C_M_RD | I2C_M_RECV_LEN transfer. That causes a kernel oops in
>> case of zero len.
>>
>> Let's check the len against zero before dereferencing buf pointer.
>>
>> This issue was triggered by syzkaller.
>>
>> Signed-off-by: Alexander Popov <alex.popov@linux.com>
>> ---
>>  drivers/i2c/i2c-dev.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/i2c/i2c-dev.c b/drivers/i2c/i2c-dev.c
>> index 036a03f..5790bc8 100644
>> --- a/drivers/i2c/i2c-dev.c
>> +++ b/drivers/i2c/i2c-dev.c
>> @@ -280,6 +280,7 @@ static noinline int i2cdev_ioctl_rdwr(struct i2c_client *client,
>>  		 */
>>  		if (msgs[i].flags & I2C_M_RECV_LEN) {
>>  			if (!(msgs[i].flags & I2C_M_RD) ||
>> +			    !msgs[i].len ||
> 
> I'd prefer
> 
> 			msgs[i].len > 0

Excuse me, it will be wrong. We stop if len is 0 to avoid the following
ZERO_SIZE_PTR dereference.

> here instead of
> 
> 			!msgs[i].len

I can change it to "msgs[i].len == 0". But is it really important?

I've carefully tested the current version with the original repro. It works correct.

> because I can parse that more easily. Semantically the patch is fine and
> can have my
> 
> Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
> 
>>  			    msgs[i].buf[0] < 1 ||
>>  			    msgs[i].len < msgs[i].buf[0] +
>>  					     I2C_SMBUS_BLOCK_MAX) {

Best regards,
Alexander